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Residue on "Cleaned" Aluminum Part Prior to Anodizing

I am having a cast aluminum (#713, Tenzalloy) part anodized. The cleaning process prior to the anodizing consists of about 20 sec in NaOH at 150 °F, rinsing with water, then 10 sec in a nitric/hydrofluoric acid (13:1, v/v) then final rinsed. When the parts come out of the NaOH they're black. When they come out of the acid some have a tan residue on them that can be rinsed off easily with a hose if done so right away. Any ideas what the black and tan stuff are?

John C. Jacco
Kingston, NY USA
2000

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The residues are smuts from alloying elements. You may need to adjust your concentrations of etching solutions to optimize etch and smut removal. -tom

  
Tom Pullizzi
Falls Township, Pennsylvania
2000

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The black stains are oxides formed during NaOH etch. The tan residue indicates either severe oxidation products and/or insufficient deoxidation with the acid.

Try lower NaOH temp (140 deg F) and longer acid immersion time.

Dado Macapagal
anodizing shop - Toronto, Ont. Canada
2000